This invention relates to ophthalmoscopic instruments and, more particularly, to a range finding device adapted for use with such ophthalmoscopic instrument.
The preliminary adjustment of an ophthalmoscopic instrument in its position with respect to an eye to be examined in an important operation. In the case of an ophthalmoscopic camera, for example, while looking for a target area near or at the pupil of the eye, the operator will first perform aligning of the optical axis of the camera to that of the eye with the help of an illuminated area on the cornea of the eye by an illuminating light beam projected from an objective lens of the camera. Then, he will turn to look through a finder of the camera for the purpose of obtaining a sharp image of the fundus of the eye, namely the retina, by moving the camera, so as to provide a suitable working distance between the camera and the eye to be examined. The term "working distance" herein used, though being generally accepted to mean the object distance for the objective lens system, is intended to refer to what is known in ophthalmoscopic art as an axial separation between the objective lens and the eye to be examined.
It is known to provide an ophthalmoscopic instrument of the type in which infrared light is made use of in illuminating an eye to be examined. With this type ophthalmoscopic camera, it is of course very difficult to achieve visual adjustment of the position and alignment of the camera relative to the eye since infrared light can hardly be viewed by the human eye. Even when the infrared image is converted to a corresponding visible image, it is difficult to achieve accurate adjustment of the working distance and also to prevent introduction of uncertainties in the later focusing procedure, since the resolving power of the infrared image converter tube is not so high as desired.
Attempts have been made to realize the condition of appropriate aligning and spacing of an ophthalmoscopic instrument of the type described to and from an eye to be examined by making use of a sight on one side of the instrument housing to correct for range in such a manner that while the cornea of the eye is illuminated by infrared light uniformly, the instrument housing is moved to bring the center of the field of view of the sight into coincidence with the pupil of the eye, for example, disclosed in U.S. Pat. No. 3,871,772. In another instrument, the sight may be otherwise used to view an image of a chart formed on the cornea. In any case, however, the visual adjusting procedure suffer from various uncertainties due to the difference among individuals in the spectral response of their eyes and it is difficult to perform because the operator is required to look through the eye-piece of the sight during that procedure.